This invention relates to gas analyzers and, more particularly, to a non-dispersive optical gas analyzer which employs multiple thermopiles as optical energy detectors.
A wide variety of non-dispersive infrared gas analyzers have been developed over the years for measuring gas concentrations in applications such as medical and pollution monitoring and industrial process control. Examples of such analyzers are disclosed in U.S. Pat. No. 3,932,754, issued Jan. 13, 1976 to M. J. Riedl, et al; U.S. Pat. No. 4,069,420, issued Jan. 17, 1978 to T. C. Ross; and U.S. Pat. No. 4,420,687, issued Dec. 13, 1983 to M. S. Martinez, et al and assigned to the assignee of the present invention.
A common feature among prior art gas analyzers of the type referred to above is the use of a single infrared detector which detects infrared radiation in the form of a train of pulses of optical energy. These pulses represent, alternately, a measure of the infrared energy absorbed by a particular gas in a sample gas mixture, and the infrared energy absorbed by a reference gas. Suitable electronic circuitry processes the detector signals, which are also in the form of a pulse train, to produce a signal proportional to the concentration of the particular gas in the sample.
The alternating pulses of optical energy are generally provided by a motor-driven chopper wheel having apertures therein. Rotation of the wheel in conjunction with the aperture placement relative to a radiation source yields the desired alternating pulses.
A primary reason for employing the alternating pulse operation is to be able to use a single detector to measure both the sample and reference gases. Prior art designs have been limited to the use of a single infrared detector because, heretofore, it has not been possible to provide multiple detectors having sufficiently closely matched performance characteristics to yield an analyzer having the required accuracy.
For example, infrared sensors are highly sensitive to small changes in ambient temperature, and the responses of two separate detectors to such changes are not necessarily the same. Thus, it has proven extremely difficult to implement a multi-detector gas analyzer capable of providing accurate measurements under conditions of changing ambient temperature, without resorting to expensive, complicated and often unreliable temperature compensating devices and circuitry.
The need for a motor-driven chopper wheel in prior art analyzers has resulted in analyzer designs which are necessarily large, require substantial amounts of power to operate, and are intolerant of high levels of shock and vibration.
Accordingly, it is an object of the present invention to provide a new and improved non-dispersive optical gas analyzer.
It is another object of the invention to provide a non-dispersive optical gas analyzer which is small in size, exhibits low power consumption, and does not require a motor-driven chopper wheel for its operation.
It is yet another object of the invention to provide a portable, low cost, highly accurate non-dispersive optical gas analyzer employing multiple optical detectors and which is operable over a wide spectrum of wavelengths from the infrared to the ultraviolet.